1. Field of the Invention
The present invention relates to an image correction method for forming correction test pattern(s) having continuously varying toner gradation on photoreceptor(s) to correct image forming condition(s), and relates to an image forming apparatus carrying out such image correction method.
2. Related Art
Gradation reproduceability which is such that tonal gradation is faithfully reproduced from highlight to shadow in reproduced images is demanded from copiers, printers, facsimile machines, and other such image forming apparatuses carrying out electrophotographic image formation processing. However, gradation in the images which are formed will vary with changes in the optical density of toner serving as developer, changes in process conditions that have been set for image formation, and so forth.
As means for improving gradation reproduceability, methods of carrying out gradation correction (γ correction) processing at times associated with certain events have therefore been adopted.
Gradation correction processing, for faithfully reproducing the tonal gradation of the image serving as original, may be carried out as follows.
A plurality of test pattern latent images, having different exposure intensities corresponding to different prescribed densities, are first formed at prescribed intervals in the paper transport direction on a photoreceptor. These test pattern latent images are then developed, at which time develop roller surface speed is held constant. Optical density of toner in the respective test pattern images produced as a result of develop is then detected, and a gradation correction curve is created based on the toner density data produced as a result of detection.
FIG. 4(b) shows examples of conventional test pattern images.
Existing test patterns (gradation patterns) ordinarily comprise on the order of three to ten (five at FIG. 4(b)) different gradation fields 101a through 101e, the respective fields that are formed being sufficiently large in size to accommodate mechanical fluctuations and so forth.
However, with such conventional test patterns, because it is sometimes the case that there will be density nonuniformities within a field due to mechanical fluctuations and so forth, density measurements are usually carried out over a prescribed region (at FIG. 4(b), circular region 102 enclosed by the dashed line) within the field. And this has resulted in the problem that density measurements are time-consuming and much toner is consumed.
An image forming apparatus has therefore been proposed (see, e.g., Japanese Patent Application Publication Kokai No. H8-211722 (1996); hereinafter “Patent Reference No. 1”) in which develop bias is continuously varied within the region of a single test pattern so as to reduce the size of the region occupied by the overall test pattern as compared with the conventional situation shown at FIG. 4(b), permitting reduction in the amount of toner consumed and reduction in the amount of time required for measurements.
In this image forming apparatus, to avoid instability in toner density when varying develop bias in stepwise fashion, photoreceptor charging potential and develop bias are continuously varied as a single test pattern, in which gradation varies continuously in the paper transport direction (scan direction), is formed. This permits the foregoing object, i.e., reduction in the amount of toner consumed and reduction in the amount of time required for measurements, to be achieved.
Now, in the aforementioned Patent Reference No. 1, correction of image forming conditions is carried out by forming a single test pattern in which gradation varies continuously and by measuring the density of this test pattern.
However, when actually carrying out image formation processing, the image forming apparatus does not produce density variation in images by varying photoreceptor charging potential or develop bias, but produces density variation in images by controlling light intensity from exposure means while holding photoreceptor charging potential and develop bias constant at appropriate values. For this reason, there has been the problem that even where correction of image forming conditions is carried out after the fashion of the aforementioned Patent Reference No. 1, correction will not necessarily be suitable or appropriate for the situation existing during actual usage.